Lithium-ion conductive garnet and method of making membranes thereof

ABSTRACT

A gallium doped garnet composition of the formula: 
       Li 7-3y La 3 Zr 2 Ga y O 12    
     where
 
y is from 0.4 to 2.0, and as defined herein. Also disclosed is a method for making a dense Li-ion conductive cubic garnet membrane, comprising one of two alternative lower temperature routes, as defined herein.

This application claims the benefit of priority under 35 U.S.C. § 119 ofU.S. Provisional Application Ser. No. 62/064,605 filed on Oct. 16, 2014,the content of which is relied upon and incorporated herein by referencein its entirety.

The entire disclosure of any publication or patent document mentionedherein is incorporated by reference.

BACKGROUND

The disclosure relates to a lithium-ion cubic conductive garnetcomposition methods of making membranes thereof.

SUMMARY

In embodiments, the disclosure provides a lithium-ion cubic conductivegarnet composition including doped variants thereof, and methods ofmaking membranes thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

In embodiments of the disclosure:

FIGS. 1A and 1B show schematic diagrams of prior art Li-metal batterystructures: single cell (FIG. 1A; 100); and double cell (FIG. 1B; 150),including, for example, a battery cover (105), a Li metal anode (110), asolid electrolyte (115), an application media (120) such as air, water,or sea water, and an external cathode (125), that is outside thebattery.

FIG. 2 shows an SEM image of a product powder or ash obtained from thedisclosed non-flame combustion process. The particle size was about 20nm. These particles are loosely connected to form two dimensionalstructures arising from the bubble films formed during heating theprecursor solution.

FIG. 3 shows an SEM image of a powder produced by non-flame-combustionand after heating to 700° C. for 4 hours.

FIG. 4 shows an X-ray diffractive spectrum of an Ga-doped garnet powderformed in the disclosed non-flame-combustion method having a 97% cubicgarnet content and after being calcined at 800° C. for 5 hrs.

FIG. 5 shows a schematic of the apparatus (500) for non-flame combustionused to make nano-garnet powder. including: a container (505) such as abeaker, bucket, or like container; a heating mantle (510) air bath, orlike heating device including means to control the temperature; aprecursor solution (507) containing source reactants, that is, beforereaction; a temperature meter (520); and a thermocouple (525).

FIG. 6 shows XRD measured garnet phase development in a Ga-dopednano-sized powder when the powder was heated to the indicatedtemperatures with the temperature ramping rate at 200° C./hr.

FIG. 7 shows XRD measured cubic garnet phase development in a Ga-dopednano-powder (squares) and a micro-powder garnet cubic (triangles) atdifferent calcining temperatures.

FIG. 8 shows XRD measured phase development of Al-doped LLZ garnetprecursors to compare the garnet phase formation through nano-powder andmicropowder routes.

FIGS. 9A to 9C show TGA/DSC measurements for a Ga-doped LLZ garnetprecursor during calcination to form garnet.

FIGS. 10A to 10C show cross-section SEM images of a Ga-doped LLZ pelletmembrane fired with a fast schedule to 1180° C.: a fractured surface(10A), and a polished surface at different magnifications (10B and 10C).

FIG. 11 shows SEM images of the polished cross-section of a Ga-doped LLZmembrane. The nominal molecular formula of the precursor isLi_(6.41)La₃Zr₂Ga_(0.6)Ox. The grain size was larger than 600 microns.

FIGS. 12A to 12C show cross-section SEM images of a Ga-doped LLZ pelletfired with a fast schedule to 1180° C. including: a fractured surface(12A); and a polished fracture surface at different magnifications (12Band 12C).

FIG. 13 shows an AC impedance curve measured from a pellet membranesample made from Li_(6.41)La₃Zr₂Ga_(0.6)O_(x) powder. The ionicconductivity calculated from this curve was 1.7×10⁻³ S/cm.

FIG. 14 shows a flow chart of the steps of the disclosed process (1400)and the steps of a comparative process (1470) (a conventional solidstate reaction process).

FIG. 15 shows a schematic in cross section view of the reactor (1500)configuration including a Pt container (1505) for sintering a cubicgarnet pellet (1510) to a dense membrane (1510) isolated from the wallsof the container by a burying powder (1520).

FIG. 16 shows a collection of Li-ion conductivity versus sinteringtemperature results from garnet membrane samples made by the disclosednano-route.

FIG. 17 shows an overlay of XRD patterns for the nano-garnet precursors,and products of the disclosure.

DETAILED DESCRIPTION

Various embodiments of the disclosure will be described in detail withreference to drawings, if any. Reference to various embodiments does notlimit the scope of the invention, which is limited only by the scope ofthe claims attached hereto. Additionally, any examples set forth in thisspecification are not limiting and merely set forth some of the manypossible embodiments of the claimed invention.

Definitions

“Non-flame combustion,” “flameless combustion,” or like terms refer, forexample, to a chemical reaction accomplished by heating in the absenceof a flame.

“Membrane” or “pellet” or like terms refer, for example, to a solidelectrolyte component, which is part of the exterior walls of a lithiumion battery cell or like articles.

“Carbon source” or “carbon precursor” or like terms refer, for example,to “carbohydrate,” “saccharide,” and like terms, includingmonosaccharides, disaccharides, oligosaccharides, and polysaccharides.In general, the monosaccharides and disaccharides, which are smaller(lower molecular weight) carbohydrates, are commonly referred to assugars.

“Sinter” or like terms refer, for example, to cause to become a coherentmass by heating without melting.

“Calcine,” “calcination,” or like terms refer, for example, to heat to ahigh temperature but without fusing to drive off volatile matter or toeffect changes.

“Firing,” “fire,” or like terms refer, for example, to the process ofmaturing ceramic products by the application of heat.

“About” modifying, for example, the quantity of an ingredient in acomposition, concentrations, volumes, process temperature, process time,yields, flow rates, pressures, viscosities, and like values, and rangesthereof, or a dimension of a component, and like values, and rangesthereof, employed in describing the embodiments of the disclosure,refers to variation in the numerical quantity that can occur, forexample: through typical measuring and handling procedures used forpreparing materials, compositions, composites, concentrates, componentparts, articles of manufacture, or use formulations; through inadvertenterror in these procedures; through differences in the manufacture,source, or purity of starting materials or ingredients used to carry outthe methods; and like considerations. The term “about” also encompassesamounts that differ due to aging of a composition or formulation with aparticular initial concentration or mixture, and amounts that differ dueto mixing or processing a composition or formulation with a particularinitial concentration or mixture.

“Optional” or “optionally” means that the subsequently described eventor circumstance can or cannot occur, and that the description includesinstances where the event or circumstance occurs and instances where itdoes not.

The indefinite article “a” or “an” and its corresponding definitearticle “the” as used herein means at least one, or one or more, unlessspecified otherwise.

Abbreviations, which are well known to one of ordinary skill in the art,may be used (e.g., “h” or “hrs” for hour or hours, “g” or “gm” forgram(s), “mL” for milliliters, and “rt” for room temperature, “nm” fornanometers, and like abbreviations).

Specific and preferred values disclosed for components, ingredients,additives, dimensions, conditions, times, and like aspects, and rangesthereof, are for illustration only; they do not exclude other definedvalues or other values within defined ranges. The composition andmethods of the disclosure can include any value or any combination ofthe values, specific values, more specific values, and preferred valuesdescribed herein, including explicit or implicit intermediate values andranges.

Fast Li-ion conductive solid electrolytes are of interest forapplication in Li-metal batteries and Li-ion batteries. In Li-metalbatteries, solid electrolyte membranes are part of the outside walls ofthe battery cells as shown in the schematic drawings of single- anddouble-cell Li-metal battery structures, respectively, in FIGS. 1A and1B, respectively. The solid electrolyte materials are expected to bechemically and electrochemically stable to Li metal, and to theapplication media, such as air, water, or sea-water. Solid electrolytesare also considered as an alternative to the commonly used organic andpolymer electrolytes in Li-ion batteries to eliminate the potentialissues of flammability and operating temperature limitations of theorganic and polymer electrolytes. Among the solid electrolyte materials,Li-oxide garnets (“LLMO” in general), particularly Li₇La₃Zr₂O₁₂(“LLZ”),^(1 to 19) are of interest due to their high Li-ion conductivity(10⁻⁴ S/cm) and high chemical stability with Li metal, and largeelectrochemical window (9V).

There are several obstacles to making a dense structure of the abovementioned garnet membranes and using them in a battery. These garnetoxides have high sintering temperatures, usually above 1200° C., duringwhich a significant loss of Li can occur. Special processes, such asusing a mother powder to bury the synthesized membrane/pellet,^(7, 9) orusing Pt to surround or encapsulate the membrane/pellet^(4, 10) duringsintering become necessary. However, using the mother powder forcovering during calcination often causes sintering of the mother powderonto the membrane/pellet. A post-calcination polishing is often needed.When using Pt in covering, the Pt can migrate into the membrane orpellet, which can cause an electrical short circuit. A composition,method of making, or both, that allows the garnet oxides to sinter at alower temperature would be highly advantageous.

LLMO garnet has two structures: tetragonal and cubic. The cubic garnethas a higher Li-ion conductivity than the tetragonal garnet.⁴ However,in reported researches, the cubic structure has to be made attemperatures above 1200° C. Aluminum (Al) is often doped into garnet tostabilize the cubic phase.^(6, 9, 12, 13) The doped Al substitutes forpart of Li in garnet. Other doping studies have been reported, such asGa doping (substitute for part of the Li^(11, 15)), Nb, Y, Sb, W, and Tedoping (substitute for part of the Zr and Li^(8, 10, 16, 18, 21)) and Srdoping (substitute for part of the La¹⁷). US 2013/0230778 disclosedgarnet compositions Li_(7-y)La_(3-x)AxZr_(2-y)M_(y)O₁₂, where 0≤x≤3,0≤y≤2, “A” can be selected from Y, Nd, Sm, and Gd; and M can be selectedfrom Nb or Ta. A method of making or composition that allows formationof a highly conductive cubic structure would also be highlyadvantageous.

Prior methods of making garnet membranes/pellets, used solid-statereaction and the Pechini process.^(1 to 19) A two-step calcination isusually used. In these processes, the precursor powder was firstcalcined at from 900 to 1200° C. to form cubic or tetragonal garnet. Thepowder was then ground and pressed into pellets. The pellets underwent asecond calcination to about 1200° C. to form a dense cubic garnet. Inthe first calcination, volatile materials such as CO₂, H₂O, etc., weredriven out of the powder, which helped to form a dense structure in thesecond calcination. In making garnet materials, workers have looked tonano-material processing method, such as the Pechini process, which is amodified sol gel process,^(5, 7, 16) or a polymerized complex method,¹⁵or a co-precipitation method. It is expected that the nano-materials canlower the sintering temperature so that one can make a dense membrane ata lower temperature. Lower processing temperature prevents losing toomuch Li from the materials. However, so far, besides using hot pressing,lower processing temperature has not been successful in making a densemembrane at temperature below 1200° C.

In embodiments, the present disclosure provides a gallium doped garnetcomposition of the formula:

Li_(7-3y)La₃Zr₂Ga_(y)O₁₂

wherey is from 0.4 to 2.0, more preferably from 0.5 to 2.0, even morepreferably from 0.6 to 2.0, and still more preferably from 0.66 to 2.0,the formula can be determined by, for example, ion coupled plasma (ICP),andthe cubic phase of the composition can be determined by, for example,x-ray diffraction (XRD) to have a lattice constant, i.e., an XRDparameter, of 13.045 Angstroms compared to a lattice constant of 12.975Angstroms obtained by firing the same cubic phase composition at above1100° C.

In embodiments, the composition was obtained by low temperature firingat from 800 to 1000° C., such as exemplified by the second heating inRoute B mentioned below.

In embodiments, y can be, for example, from 0.4 to 2.0.

In embodiments, the sintering temperature, i.e., the second heating ineither Route A or Route B, of the composition is below 1000° C., forexample, about 950° C. The literature has reported sintering garnetdoped with other than Ga above 1150° C., see for example, R. Murugan, etal., High conductivity yttrium doped Li₇La₃Zr₂O₁₂ cubic lithium garnet,Electrochem. Comm. 13 (2011) 1373. A Ga-doped garnet has been reportedto have a sintering temperature of 1080° C., but the report used adifferent method for sample preparation (see El Shinawi, et al., J.Power Sources, 225 (2013) 13-19).

In embodiments, the Li can be substituted by Ga with up to 50 mol %, toprovide, for example, a compound of the formula Li₄La₃Zr₂Ga₁O₁₂ (i.e., 1Ga atom substituted for 3 Li atoms). In embodiments, the La can besubstituted with Y up to 30 mol % to provide, for example, a compound ofthe formula Li₇La₂₇Y_(0.3)Zr₂O₁₂ (i.e., 1 Y atom substituted for 1 Laatom). In embodiments, the Zr and Li can be substituted by Nb with up to100 mol % to provide a compound, for example, of the formulaLi₆La₃ZrNbO₁₂ (i.e., 1 Nb substituted for 1 Zr and 1 Li).

In embodiments, the present disclosure provides two related butdifferent methods of making a Li-ion conductive cubic garnet membrane,referred to as Route A and Route B.

In embodiments, the present disclosure provides a method of making(Route A) a Li-ion conductive cubic garnet membrane, comprising:

forming a nitrate source aqueous solution comprised of a first nitratesource and a nitrate dopant source;

contacting the nitrate source aqueous solution and a carbohydrate sourcein mole ratio of from 1:1 to 1:4, and a first heating from 200 to 550°C., to form a nano-particle ash (The first heating can begin at, forexample, about 200° C., and the solution begins to foam, and starts tocombust (no-flame). The upper end of the first heating range of 550° C.is the top temperature during the flameless combustion);

a second heating (Route A)(1410) of the nano-particle ash at from 650 to700° C. to produce a garnet nanoprecursor having nano-sized particles offrom 10 to 100 nm;

pelletizing the garnet nanoprecursor to form a Li-ion conductive cubicgarnet pellet; and

a third heating (1450) of the pelletized garnet pellet at a toptemperature of from 950 to 1200° C., at from 0.5 to 30 hrs, to form adense membrane of the Li-ion conductive cubic garnet.

In embodiments, the first nitrate source comprises a mixture of each ofLiNO₃, La(NO₃)₃, and ZrN₂O₇; the nitrate dopant source can be selectedfrom at least one element from the Groups IIA to VIIA, and IIIB to IVBfrom the periodic table of elements; the carbohydrate source can havefrom 5 to 10 carbon atoms; and the nano-particle ash has a size of from10 to 50 nm.

In embodiments, the carbohydrate source can have, for example, from 5 to10 carbon atoms. In embodiments, the carbohydrate source can be selectedfrom, for example, a carbohydrate, a complex sugar, a simple sugar, adisaccharide such as sucrose, a monosaccharide, or a combinationthereof.

In embodiments, the dense membrane of the Li-ion conductive cubic garnethas a Li-ion conductivity of from 0.2×10⁻³ to 2.0×10⁻³ S/cm.

In embodiments, the third heating of the pelletized garnetnano-precursor (i.e., nano-sized particles) is held at 800° C. for 2 to6 hrs to form a pure cubic garnet phase and to stabilize the pure cubicgarnet phase.

In embodiments, the third heating can be accomplished, for example, in aclosed platinum vessel, and the pellet can be surrounded, for example,in a garnet burying powder selected from at least one micrometer sizedLi-oxide garnet.

The garnet burying powder can preferably be doped so that the buryingpowder has a cubic phase and a high sintering temperature. An Al-dopedgarnet is an example. The burying powder is different from the membranepowder in particle size. The burying powder has a particle size (D₅₀) offrom 1 to 10 micrometers. The powder for making a membrane has aparticle size (D₅₀) of from 30 to 700 nanometers The burying powder isalso a different material from the membrane powder because of differentdoping.

In embodiments, the nitrate dopant source is selected from at least onea metal nitrate, or a mixture of at least two (i.e., two or more)different metal nitrates.

In embodiments, suitable metal nitrates can include, for example,Ga(NO₃)₃, Al(NO₃)₃, and like metal nitrates, or mixtures thereof.

In embodiments, the nitrate dopant can be generated in situ from a(e.g., by combining any appropriate reactants such as a metal and nitricacid. A suitable nitrate dopant source can be, for example, a Mo metaland nitric acid.)

In embodiments, the method of making via Route A can further compriseadding from 1 to 20 wt % excess of a nitrate source containing Li, forexample, LiNO₃, to compensate for the loss, e.g., by evaporation, of Liduring the third heating.

In embodiments, the Li-ion conductive cubic garnet produced can be acomposition of the formula, for example:

Li₇La₃Zr₂O₁₂

where any of the Li, La, Zr, or a combination thereof, is partially orfully substituted by at least one dopant selected from at least oneelement from Group IIA to VIIA, IIIB to IVB, or a combination thereof.As example, Li can be substituted up to 50 mol %, La can be substitutedup to 30 mol %, and Zr can be substitute up to 100 mol %.

In embodiments, the disclosure provides a method of making a Li-ionconductive cubic garnet referred to a Route B, comprising:

forming a nitrate source aqueous solution comprised of a first nitratesource and a nitrate dopant source;

contacting the nitrate source aqueous solution and a carbohydrate sourcein mole ratio of from 1:1 to 1:4, and a first heating of the mixturefrom 200 to 550° C., to form a nano-particle ash;

a second heating (Route B)(1420) of the nano-particle ash at from 775 to1000° C. to produce a Li-ion conductive pure cubic garnet powder havingmicron-sized particles having irregular shape;

milling the cubic garnet powder to produce a sub-micron powder;

pelletizing the sub-micron powder to form a garnet pellet; and

a third heating of the garnet pellet at from about 950 to about 1200° C.to form a dense Li-ion conductive cubic garnet membrane.

In embodiments, the third heating can be accomplished, for example, in aclosed platinum vessel, and the pellet can be surrounded, for example,in a burying powder selected from at least one micrometer sized Li-oxidegarnet.

In embodiments, the first nitrate source can comprises a mixture ofLiNO₃, La(NO₃)₃, and ZrN₂O₇; the nitrate dopant source can be selectedfrom at least one element from the Groups IIA to VIIA, and IIIB to IVB;the carbohydrate source can have from 5 to 10 carbon atoms; and thecubic garnet powder can have irregularly shaped and porous particlesbefore milling, and the cubic garnet powder can have sub-micron-sizedparticles of from 100 nm to 1000 nm after milling.

In embodiments, the carbohydrate source can have, for example, from 5 to10 carbon atoms. In embodiments, the carbohydrate source can be selectedfrom, for example, a carbohydrate, a complex sugar, a simple sugar, adisaccharide, a monosaccharide, or a combination thereof.

In embodiments, milling can be accomplished by, for example, jet millingor attrition milling, to provide submicron-sized particles such as about600 nm. In embodiments, milling to a particle size of from 200 to 600 nmis preferred.

In embodiments, the nitrate dopant can be generated in situ from anitrate dopant source (e.g., by combining any appropriate reactants suchas a metal or, metal salt, and nitric acid). In embodiments, the nitratedopant can be selected from, for example, at least one metal nitrate, ormixtures thereof. Suitable metal nitrates can include, for example,Ga(NO₃)₃, Al(NO₃)₃, and like metal nitrates, or mixtures thereof. Asuitable nitrate dopant source can be, for example, a Mo metal and asource of nitric acid.

In embodiments, the method of making of Route B can further compriseadding from 1 to 20 wt % excess of a nitrate source containing Li, forexample, LiNO₃, to compensate for the loss, e.g., by evaporation, of Liduring the third heating.

In embodiments, the dense membrane can have a Li-ion conductivity of,for example, from 0.2×10⁻³ to 2.0×10⁻³ S/cm.

In embodiments, the as-prepared ash can be milled to break downagglomerates, which have a two-dimensional flake shape.

In embodiments, the second heating of the pelletized Li-ion conductivecubic garnet, e.g., garnet oxides, can be accomplished, for example, atfrom 0.5 to 30 hrs, of the second heating at the top temperature of atfrom 650 to 700° C. at from 775 to 1000° C., i.e., a lower sintertemperature compared to the prior art.

In embodiments, the dense membrane can have a Li-ion conductivity ofabout 0.2×10⁻³ S/cm to 2×10⁻³ S/cm, e.g., above 2×10⁻⁴, such as 1×10⁻³S/cm.

In embodiments, the cubic garnet has a Li-ion conductivity of, forexample, from 0.5 to 1.7×10⁻³ S/cm. The garnet has a conductivity, whichis different from the dense membrane, for example, above about 5×10⁻⁴S/cm.

In embodiments, the dense membrane can have a grain size of, forexample, from 1 to 100 microns, from 1 to 50 microns, from 1 to 10microns, and like values, including intermediate values and ranges.

In embodiments, the dense membrane can have a grain size of, forexample, from 100 to 600 microns. The upper limit of the grain size isconfined by the membrane thickness.

In embodiments, the disclosure provides garnet composition to controlthe size of the grains in the dense membrane, for example, a densemembrane having a Li concentration of from 10 to 30 mol % less than thatin a compound of the formula Li₇La₃Zr₂O₁₂.

In embodiments, the garnet can have, for example, a pure cubic phase.

In embodiments, the nitrate precursor can be selected from, for example:LiNO₃, La(NO₃)₃, ZrN₂O₇, and mixtures thereof.

In embodiments, the method can further comprise, for example, at leastone of:

adding a nitride dopant prior to contacting the aqueous solutions;

generating a nitride dopant in situ by, for example, reaction of a metalor a metal oxide with nitric acid;

or combinations thereof,

and the dopant being selected from at least one of Ga(NO₃)₃, Al(NO₃), ormixtures thereof.

In embodiments, the above mentioned metal nitrate precursors can beformed, for example, in situ, in the solution by reaction of a metal ora metal oxide with nitric acid.

In embodiments, the carbohydrate source can be selected, for example,from a carbohydrate, a complex sugar, a simple sugar, e.g., sucrose,lactose, maltose, etc., a disaccharide, a monosaccharide, or acombination thereof.

In embodiments, the carbohydrate source can have, for example, from 5 to10 carbon atoms.

In embodiments, the carbohydrate source can be, for example, sucrose.

In embodiments, the present disclosure provides a method of making acubic garnet powder, comprising:

a first heating of the as-prepared ash to 800° C. for about 2 to 6hours, for example, 4 hours.

In embodiments, the present disclosure provides a method of making acubic garnet membrane, comprising:

pelletizing the abovementioned Li-ion conductive cubic garnet; and

accomplishing a second heating of the pelletized Li-ion conductive cubicgarnet at from about 950 to about 1200° C. to form a dense membrane.

In embodiments, the second heating can be accomplished, for example, ina closed platinum vessel, and the pelletized Li-ion conductive cubicgarnet membrane can be surrounded in, for example, a burying powder.

In embodiments, the burying powder can be selected, for example, from atleast one micrometer sized Li-oxide garnet powder.

In embodiments, the micrometer size Li-garnet burying powder can beselected from, for example, high melting temperature Li-oxide garnet,e.g., Al-doped Li-oxide garnet.

In embodiments, the disclosure provides a method of making a Li-ionconductive membrane from the cubic garnet by a nano-material route.

In embodiments, the disclosure provides a non-flame combustion methodfor making a garnet membrane via a nano-material process. By using anon-flame combustion approach, it is possible to make nano-size garnetprecursors. These precursors can then be pressed into pellets andcalcined to make a membrane directly. A fully sintered membrane can bemade by firing at 950° C. Such membranes can have an ionic conductivityas high as, for example, 1.7×10⁻³ S/cm.

In embodiments, the disclosed lower processing temperatures minimizes oreliminates the loss of lithium.

The nano-sized starting materials can be made by, for example, anon-flame combustion method. In the non-flame combustion method, anitrate precursor or nitrate source reacts with a suitable carbonsource, for example, a sugar, such as sucrose, at a nitrate sourcesolution to carbon source solution molar ratio of from 1 to 4, andpreferably from 2 to 3. With these ratios, the reaction induces a mildnon-flame combustion of the reactants, leaving an ash in nano-particleform. The particle size is, for example, about 20 nm. After a briefpost-combustion-calcination to 700° C., all remnant combustiblematerials can be removed from the ash, while the nano-particles canstill retain their size at about 50 nm. The resulting nano-powder can beused to directly make a garnet membrane.

In embodiments, in a second or later sintering process, the well mixednano-garnet starting materials permit forming of a cubic phase garnet ata much lower temperature (e.g., at about 800° C.) compared tomicron-size starting materials in a solid state reaction (e.g., at about1100 to 1200° C.). In embodiments, the disclosure provides a method ofmaking a garnet membrane having a significant firing processmodification, which modification prevents Li-loss and also keeps theLi-ion conductive membrane intact. With different doping, theLi₇La₃Zr₂O₁₂ (LLZ) garnet can have different sintering temperatures.This sintering temperature can also be modified with particle size. Ahigher sintering temperature LLZ-garnet, for example, Al-doped LLZgarnet in micro-size, which is found to sinter at 1230° C., can be usedas a burying powder to cover the membranes during firing. The membranecan be made from low temperature sintered materials, for example,Ga-doped LLZ garnet, which starts to sinter, for example, at 950° C. Thefiring temperature schedule is significant for obtaining pure cubicgarnet at lower temperatures, such as 800 to 1,000° C.

In embodiments, the disclosed process is advantaged in several aspects.

The non-flame combustion process produces significantly smaller sizeparticles (e.g., about 20 nm) compared to the Pechini process (amodified sol gel process, that produces particles of about 100 nm).Smaller particles are more reactive, resulting in forming garnet at muchlower temperature. The experimental results of the disclosure show that,for example, the Ga-doped LLZ nano-sized starting materials made bynon-flame combustion can form a near pure (e.g., having a purity of from95 to 100%) cubic garnet at 800° C. A Pechini process using garnetstarting materials formed tetragonal garnet at 900° C., and cubic garnetat above 1100° C. Compared to a micron-sized powder solid statereaction, garnet (all in cubic phase) is often formed at above 1200° C.

In embodiments, the disclosed non-flame-combustion method of Route Aproduced a powder product that can be used to make a membrane directly(unlike the Pechini process and the micro-powder process, in which thepowder has to be heated to 900 to 1200° C. to allow the precursor tofully react to remove all volatile materials, and then the garnet powderis ground). The garnet powder product is then pelletized to pellets, andthen subjected to a second firing to about 1200° C. to form denselysintered pellets or also know as a membrane or pellet membrane.

The disclosed non-flame-combustion made nano-powder can be used to formdifferent size grain membranes by adjusting the composition and firingconditions. The adjustable grain size can be, for example, in a smallersize range of from 1 to 100 microns, or preferably from 1 to 20 microns,or in a larger size range of from 100 to 600 microns. For large-sizegrain membranes, one grain may cover the whole thickness of the pressedpellets (i.e., the membrane; see FIG. 11).

The disclosed nano-particle ash made by non-flame-combustion can besintered to form a dense membrane at about 950° C., and the densemembrane has satisfactory Li-ion conductivity of, for example, about1×10⁻³ S/cm.

The collection efficiency of the disclosed non-flame-combustionnano-particle production process is about 100%.

High calcination temperatures for densifying the LLZ membranes can causesignificant loss of Li. To prevent loss of Li, the mother powder of themembrane is usually used to bury the membrane during the calcination.However, after calcine, the mother powder can stick onto the membraneand require a polishing treatment to remove the mother powder from themembrane. In the disclosed method of making, a nano-powder was used tomake the membrane, and a micron-size powder was used as the buryingpowder. In the disclosed method of making, differently doped LLZ garnetpowders were used, which doped powder has a higher sinteringtemperature, to cover the membrane that has a lower sinteringtemperature. For example, an Al-doped micron-sized LLZ powder was usedto cover a membrane made of Ga-doped LLZ powder or precursors. Theburying powder can be easily brushed-off from the membrane after calcineand without the need for removal of micron sized mother or buringpowder.

General Preparative Method

Li-ion conductive garnet materials or membranes were prepared using anon-flame combustion method. In this method, nitrate precursormaterials, usually nitrates, such as LiNO₃, La(NO₃)₃, ZrN₂O₇, and likecompounds, were dissolved in water to form a clear solution with totalmolar concentration of about 3 M. The carbon source or carbohydratecompound, for example, a sugar, such as sucrose, was combined such asadded into the nitrate precursor solution with a molar concentrationratio of from 1 to 4 times the total nitrate precursor molarconcentration. A preferred amount of carbon source such as sucrose was amole ratio, for example, of about 1 to 4 times, and about 2 to 3 timesthe total nitrate precursor, such as 2 to about 4 M. Carbon sourcesolutions were preferably prepared at or near saturation to minimizeexcess water and water removal. The combined solution was then heated toevaporate some of the water. With a further increase in the temperature,the solution began to foam and turn brown. At temperatures of about 200°C., the reaction between nitrates and sucrose began. The reaction wasaccompanied by non-flame combustion, and eventually left an ash powderin the reactor. The highest temperature during the combustion was about550° C. Some free carbon remains in the powder making the powder appeargrey color.

FIG. 2 shows the SEM of the as-combusted powder. The particle size wasabout 20 nm. These particles are loosely connected to each other andform two dimensional structures, which arise from the bubble filmsformed during the heating the precursor solution.

During the non-flame combustion, only gasses were given off. Thesegasses were mainly CO₂ and N₂, with small amounts of CO or NO_(x). Thereshould be no loss of metal elements, considering that the highesttemperature reached during combustion is about 550° C. The theoreticalmetal oxides collection efficiency should be near 100%.

By heating the as-combusted powder to 700° C., one can remove theremaining carbon. In the disclosed methods, the powder in alcohol wasfirst ball milled for 24 hrs to break up the loose agglomerates, andthen the solution was dried to obtain a powder. The powder was thenheated to 700° C. for 4 hours. After this calcination, the powder turnspure white. FIG. 3 is an SEM image of the powder after 700° C.calcination showing that the particle size increased to about 50 nmafter the 700° C. heat treatment.

Doping of the garnet crystal can stabilize the cubic phase, which hasshown higher Li-ion conductivity than the tetragonal phase garnet. Innon-flame-combustion, the doping can be done by adding the dopingelement precursors into the precursor solution. For example for Aldoping, a certain amount of Al(NO₃)₃ was used. The certain amount wasdetermined depending on the specific empirical or molecular formula ofthe particular compound being prepared, e.g., using an exactstoichiometric or mole equivalent, or a slight excess of the dopant tobe substituted into the preceding compound. For Ga doping, a certainamount of Ga(NO₃)₃ was used. The doped powder can form a stable cubicphase. Nano-size materials allow the solid reactions to occur at muchlower temperatures. For example, a Ga-doped powder can form 97% cubicgarnet phase when heated to 800° C. for 4 hrs, see the XRD in FIG. 4.The cubic phase can remain stable to higher temperatures, see FIG. 7.

EXAMPLES

The following examples demonstrate the differences between the garnetformed through the disclosed nano-material Route A and the disclosedmicro-material Route B. The following examples also provide the Li-ionconductivities of the prepared garnet membranes.

Example 1

Comparison of Ga-Doped LLZ Garnet Formation Temperature forNano-Materials and Micro-Materials.

Ga-doped Li-oxide garnet was made by two method: the disclosed non-flamecombustion method, and a comparative method used in a conventional solidstate reaction (SSR) method. In the non-flame combustion method, LiNO₃,La(NO₃)_(3.6)H₂O, ZrN₂O₇.xH₂O, and Ga(NO₃)₃ were dissolved in water witha molar ratio of 7.0:3.1:2.0:0.8 to form a clear solution having a totalconcentration of 2.9 M. Sucrose a the carbon source, having two times(2×) the number of moles of the total nitrate precursor moles was addedto the solution, for example, nitrate precursor moles was 3.1 and thesucrose moles was 6.2. Next, about 50 mL of the solution of nitrateprecursor and sucrose was poured into a 600 mL beaker, which was mountedin a heating mantle, see for example, FIG. 5. The solution was mildlyheated to evaporate the water. With the increase of the temperature withgradual concentration, the solution foamed and turned brown. At around200° C., the reaction between the nitrate precursor and sucrose started,accompanied with non-flame combustion. The highest temperature duringthe combustion was about 550° C. After the non-flame combustion, about 5g of the residual powder was collected. ICP measurement showed that thispowder had a nominal molecular formula of Li_(6.41)La₃Zr₂Ga_(0.6)O_(x).

In a comparative conventional solid state reaction (SSR) method, theprecursors were Li₂CO₃, La₂O₃, ZrO₂, and Ga₂O₃. These precursors weremixed at molar ratio of 6.5:3.0:2.0:0.58, and then heated to about 1200°to allow the precursors to react to form garnet.

For nano-powder, the firing schedule was significant to forming thecubic phase garnet. FIG. 6 shows the phase transformation of Ga-dopednano-powder by firing the powder to different temperatures with aramping speed of 200° C./hr. At 800° C., the nano-powder fully reactsand forms 97% cubic phase garnet, with small amounts of LiGaO₃impurities. However, when the powder was heated to 900° C., a tetragonalphase developed and became the dominate phase. This tetragonal phasecompletely shifts back to a cubic phase at 1100° C.

FIG. 7 shows the XRD measured garnet phase development with temperaturefor both nano-powder and micro-powder. In the nano-powder firing, whenfiring to 800° C. and higher, the firing was maintained at 800° C. for 4hours, and then continued to ramp to higher temperatures with a rate of100° C./hr. From FIG. 7, the nano-powder reacts and forms cubic garneteven at 700° C. About 17 wt % cubic garnet was detected after the 700°C. heat treatment. The powder reacts completely and forms almost purecubic garnet at 800° C. A trace amount of LiGaO₂ was observed. With thisfiring schedule, the cubic phase was retained in higher temperaturefiring. The micro-powder reacts slowly, and the cubic garnet increasegradually with temperature until 1100° C. to form pure cubic garnetphase. In both powder, no tetragonal garnet was observed in any stage ofthe firing.

In the nano-powder firing to above 800° C., the firing remained at 800°C. for at least 4 hours, and was then increased to higher temperatures.FIG. 7 shows a graph of cubic garnet weight percent versus calcinetemperature for samples made by the disclosed nano-route and compared tosamples made by the micro-powder route. The low temperature cubic garnetwas only observed with nano-powder synthesis route. The disclosedsintering method at from 800° C. to 1000° C. provides a stable lowtemperature cubic garnet. The low temperature garnet was observed withother nano-processes at about 800° C., but it was assumed to beunstable, and converted into tetragonal garnet when the temperature wasincreased, such as to 900° C. The results indicate that the conductivityof the low-temperature cubic garnet is accessible when the nano-powderis sintered at low temperature.

Example 2

Comparison of Al-Doped Garnet Formation Temperature for Nano-Materialsand Micro-Materials

Al-doped Li-oxide garnet was made by two methods: one method used thedisclosed non-flame combustion, and the other method used a conventionalsolid state reaction (SSR). In the non-flame combustion method, LiNO₃,La(NO₃)₃6H₂O, ZrN₂O₇.xH₂O and Al(NO₃)₃9H₂O were dissolved in water withmolar ratio of 7.55:3.13:2.0:0.24 to form a clear solution having totalconcentration of 3.0 M. Sucrose having a mole amount two times (2×) thetotal precursor moles was added to the solution. After the non-flamecombustion preparation, the obtained powder had a nominal molecularformula of Li₆₂La₃Zr₂Al_(0.2)O_(x), measured by ICP.

FIG. 8 shows the XRD measured phase development of the Al-doped LLZnano-powder, compared with a same composition micro-powder made by SSRprocess. The Al-doped nano-powder formed about 87% cubic phase at 800°C., followed with a partial shift of the phase to tetragonal, and thenchanged back to 100% cubic phase at 1000° C. A temperature hold at 800°C. for 4 hours did not prevent the garnet phase shifting to tetragonalat 900° C. The same composition micro-powder developed cubic phasegarnet gradually without tetragonal phase at any temperature, and formednear 100% cubic phase at 1200° C.

Example 3

Thermal Analysis (TGA/DSC) was used to measured the differences in thereaction temperature for nano- and micro-materials.

FIGS. 9A to 9C show TGA/DSC measurements for a Ga-doped LLZ garnetprecursor during calcination to form garnet: a nano-powder LLZ-15prepared by non-flame combustion with nominal molecular formula ofLi_(6.0)La₃Zr₂Ga_(0.2)Ox (9A); a nano-powder LLZ-17 prepared bynon-flame combustion with a nominal molecular formula ofLi_(6.41)La₃Zr₂Ga_(0.6)Ox (9B); and a micro-powder precursor with anominal molecular formula of Li_(6.41)La₃Zr₂Ga_(0.6)Ox, the same asLLZ-17 (9C). The heat ramp for all three TGA/DSC profiles was 10° C./minto 1200° C. in air and in platinum lined aluminum pans.

FIGS. 9A, 9B, and 9C show TGA/DSC measurements of two nano-powders(LLZ-15, with nominal molecular formula of Li_(6.0)La₃Zr₂Ga_(0.2)Ox, andLLZ-17, with nominal molecular formula of Li_(6.41)La₃Zr₂Ga_(0.6)Ox anda micro-powder (LPG) having the same nominal molecular formulaLi_(6.41)La₃Zr₂Ga_(0.6)Ox as the LLZ-17. The measurements were done byheating the powder in a Pt lined pan from room temperature to 1200° C.Two endothermal peaks were observed at 730° C., and 845° C. for bothnano-powders. The 730° C. peak is from the melting of Li₂CO₃. The 845°C. peak is from the solid sate reaction, which accompanied with a weightloss peaked at about 840° C. in the DTG curves. This is the onlyreaction found for nano-powder. From XRD measurement a near 100% cubicgarnet forms at 800° C., the TGA/DSC observed reaction also indicatesformation of cubic garnet. The TGA measured weight loss was believed tobe from the CO₂ given off from the Li₂CO₃ precursor by the solid statereaction.

For the micro-powder LPG, the DTG curve shows three major weight losspeaks from about 770° C. to about 1100° C., indicating that the solidreaction involving Li₂(CO₃) in the micro-powder system occurredgradually throughout the temperature range, and finished at a muchhigher temperature compared to nano-powder. This is consistent with theXRD results, in which the formation of garnet is gradual and completedat about 1100° C. (i.e., top temperature or maximum temperature).

Example 4

Ga-Doped LLZ Garnet Membrane Calcination and its Microstructure

Ga-doped LLZ garnet membranes were made by pelletizing 1.5 g of Ga-dopedLLZ precursor nano-powder into pellets with a diameter of 28.78 mm using10 kpsi pressure. Calcination was done in a Pt tray with a tightly fitcover. Inside the box, a layer of Al-doped garnet micro-powder was lineda the bottom. The pellets were laid down on the Al-doped LLZ powder. Ontop of the pellets, another layer of Al-doped LLZ powder was spread on.This firing equipment and structure helped in preventing Li loss. Thefiring schedules were as follows:

Fast schedule RT to 900° C., 200° C./hr 900° C., hold for 2 hrs 900° C.to top temperature, 100/hr Top temperature, hold for 7 hrs Toptemperature to RT, 200° C./hr

Slow schedule RT to 800° C., 150° C./hr 800° C., hold for 4 hrs 800° C.to top temperature, 100/hr Top temperature, hold for 7 hrs Toptemperature to RT, 200° C./hrwhere “top temperature” refers to the maximum temperature for aspecified synthesis.

After the firing, the burying powder can be easily brushed-off from thepellet surface. The Al-doped LLZ garnet micro-powder sintered at 1230°C., and the Ga-doped LLZ powder sintered at 1050° C.

FIGS. 10 and 11 show the SEM cross-section images of pellets from aprecursor with a nominal molecular formula of Li_(6.41)La₃Zr₂Ga₀₆O_(x)(LLZ-17) after firing to 1180° C. (i.e., top temperature) with the fastschedule and 1050° C. (i.e., top temperature) with the slow schedule,respectively. In this precursor powder the Li was 23 relative % morethan, or in excess of, the ideal stoichiometry for the target compound.Large grains (e.g., 200 to 600 micrometers) were developed in bothpellets, especially for the slow firing, large single crystal covers thewhole thickness of the pellet. The average grain size was about 500microns.

By adjusting the garnet composition, small grains can also be obtained.FIGS. 12A to 12C show the SEM images of a pellet made from precursorshaving a nominal molecular formula of Li₆La₃Zr₂Ga_(0.2)O_(x), in whichLi was 5 relative % less than the ideal stoichiometry for the targetcompound. The average grain size was about 20 microns. FIGS. 12A to 12Cshow cross-section SEM images of this Ga-doped LLZ pellet membrane firedwith a fast schedule to 1180° C. Smaller grains of about 20 microns havebeen developed in this pellet.

Example 5

Ionic Conductivity of the Ga-Doped LLZ Garnet Membrane Made by theNano-Powder

The ionic conductivity of two Ga-doped LLZ garnet membranes weremeasured by AC impedance. To prepare for the measurement, edges of thepellet membrane samples were abraded to remove any cracks or edgedefects. The membranes were then coated with gold on both sides untileach side had a resistance of less than or equal to 5 ohms (Ω), whichwas approximately 3 to 4 microns of sputtered gold. The edges wereabraded again to remove the sputtered gold, creating a resistance pathdirectly through the membrane. At this point the membrane was tested onthe AC impedance meter to obtain the impedance spectra.

As an example, a non-flame combustion made nano-powder having a nominalcomposition of Li_(6.41)La₃Zr₂Ga_(0.6)O_(x), was first heated to 700° C.for 4 hours, and then pressed into pellets. The pellets was then firedto 1050° C. for 7 hours with the slow schedule shown in Example 4.During firing, the pellet was covered by an Al-doped garnet powder withnominal molecular formula of Li_(6.6)La₃Zr₂Al_(0.3)O_(x). FIG. 13 is theAC impedance spectra from this pellet. The pellet's surface area was0.442 cm², and the thickness was 570 microns.

FIG. 14 shows a flow chart of the steps of the disclosed process (1400)and the steps of a comparative process (1470) (a conventional solidstate reaction process). In the disclosed membrane preparation method, anano-sized cubic garnet precursor ash powder was pre-fired (pre-heated)at 700° C. to remove residual carbon. The product was pelletized and thepellet was heated initially to 800° C. and then heated further accordingto the slow heating schedule (1450) to form the disclosed dense cubicgarnet membrane.

FIG. 15 shows a schematic in cross section view of the reactor (1500)configuration including a Pt container (1505) for sintering a greensample cubic garnet pellet (1510) to a dense membrane (1510) isolatedfrom the walls of the container by a burying powder (1520). Inembodiments, the burying powder can be, for example, the micron sizedgarnet precursor with dopant, such as Al, that has higher sinteringtemperature than the garnet for making the pellet, such a Ga-dopedgarnet.

FIG. 16 shows the Li-ion conductivity measured from the Li-oxide garnetsamples made by the disclosed nano-material route. FIG. 16 showsrepresentative Li-ion conductivity versus sintering temperature resultsfrom garnet membrane samples made by the disclosed nano-route. Thesesamples each had different compositions and firing conditions. Thex-axis is the measured sintering temperature. The Ga concentrations(0.2, 0.6. 1.0 and 2.0 with respect to the nominal molecular formula ofthe garnet compound) are shown in the key by the different symbols. Forthe compositions included in the disclosure, Ga (dopant) wasinterchanged for some or all of the Li in the original powder, and theLa and Zr content was kept the same. Higher Ga doping allows themembranes to sinter at lower temperature as illustrated in the herein.

FIG. 17 shows an overlay of XRD patterns for the nano-garnet precursorsfired to 800° C. (1705)(short dashed line weight); 1,100° C. (1710)(long dashed line weight), and known cubic garnet peaks from an XRDdatabase (1715) (solid line weight). The XRD analyser uses known sets ofthe XRD peaks from their database to identify the measured XRD peaks.LaB₆ was used as an internal calibration reference (1720)(stars). Ashift of the cubic phase patterns can be seen for the low-temperatureand the high-temperature cubic garnets.

In another example, a same sample composition as above was fired to1100° C. for 6 hrs with the same temperature ramping schedule (slowschedule), the AC impedance measured conductivity was 1.4×10⁻³ S/cm.

In another example, a same sample composition as above was fired to1100° C. for 7 hrs with the fast schedule, the AC impedance measuredconductivity was 5.3×10⁻⁴ S/cm.

In another example, a non-flame combustion made nano-powder with nominalcomposition of Li_(6.0)La₃Zr₂Ga_(0.2)Ox was fired to 1100° C. for 6 hrswith the fast firing schedule, the AC impedance measured conductivitywas 4.5×10⁻⁵ S/cm.

In another example, a non-flame combustion made nano-powder with nominalcomposition of Li_(6.0)La₃Zr₂Ga_(0.2)Ox was fired to 1180° C. for 6 hrswith the fast firing schedule, the AC impedance measured conductivitywas 3.7×10⁻⁴ S/cm.

In another example, a non-flame combustion made nano-powder with nominalcomposition of Li_(6.0)La₃Zr₂Ga_(0.2)Ox was fired to 1200° C. for 6 hrswith the same temperature ramping schedule, the AC impedance measuredconductivity was 4.6×10⁻⁴ S/cm.

In another example, a non-flame combustion made a nano-powder having anominal composition of Li_(5.5)La₃Zr₂Ga_(0.52)Ox, which was fired to1050° C. for 7 hrs with the slow temperature ramping schedule. The ACimpedance measured conductivity was 2.8×10⁻⁴ S/cm. The grain size inthis membrane was 1 to 10 micrometers.

In another example, a non-flame combustion made a nano-powder having anominal composition of Li_(5.5)La₃Zr₂Ga_(0.52)Ox, which was fired to1100° C. for 6 hrs with the fast temperature ramping schedule. The ACimpedance measured conductivity was 6.5×10⁻⁴ S/cm. The grain size inthis membrane was 100 to 500 micrometers.

In another example, a non-flame combustion made a nano-powder having anominal composition of Li_(5.5)La₃Zr₂Ga_(0.52)Ox, which was fired to950° C. for 30 hrs. The AC impedance measured conductivity was 4.2×10⁻⁴S/cm. The grain size in this membrane was a mixture of 1 to 5micrometers small grains and 100 to 200 micrometers large grains.

All of the disclosed cubic garnet samples are “dense” or hermetic inthat they have little or no porosity. The apparent density of the sampleover the helium gas pycnometry measured density was above 95%. Thismeans that the open pore porosity is less than 5%. The apparent densityof the garnet sample is calculated by the sample weight divided by theapparent volume of the sample. Helium gas pycnometry measures densitywithout considering open pores.

Example 6

X-Ray Diffraction (XRD) Lattice Constant Determination for Low and HighTemperature Fired Cubic Garnet Samples.

Table 1 lists measured lattice constants for cubic garnet samplesprepared with the disclosed low temperature firing or a high temperaturefiring. The lattice constants were determined by standard XRD powdermethods. LaB₆ was added as an internal standard for accurate latticeconstant measurement.

TABLE 1 Lattice constants for low and high temperature fired cubicgarnet. Firing temperature Lattice constant Low temperature cubic garnet13.0450(1) Å (800 to 1000° C.) High temperature cubic garnet 12.9752(1)Å (greater than 1000° C.)

The disclosure has been described with reference to various specificembodiments and techniques. However, many variations and modificationsare possible while remaining within the scope of the disclosure.

1. A method of making a Li-ion conductive cubic garnet, comprising:forming a nitrate source aqueous solution comprising a first nitratesource and a nitrate dopant source; contacting the nitrate sourceaqueous solution and a carbohydrate source and heating in a firstheating step to form a nano-particle ash; a second heating step of thenano-particle ash to produce a garnet nanoprecursor; pelletizing thegarnet nanoprecursor to form a Li-ion conductive cubic garnet pellet;and a third heating step of the garnet pellet to form a dense Li-ionconductive cubic garnet membrane.
 2. The method of claim 1, wherein: thefirst nitrate source comprises a mixture of LiNO₃, La(NO₃)₃, and ZrN₂O₇;the nitrate dopant source is at least one element from Groups IIA toVIIA, and IIIB to IVB; the carbohydrate source has from 5 to 10 carbonatoms; and the nano-particle ash has a size in a range of 10 nm to 50nm.
 3. The method of claim 1, wherein the dense Li-ion conductive cubicgarnet membrane has a Li-ion conductivity in a range of 0.2×10⁻³ S/cm to2.0×10⁻³ S/cm.
 4. (canceled)
 5. The method of claim 1, wherein the thirdheating step is conducted in a closed platinum vessel, and wherein thegarnet pellet is a pellet surrounded in a garnet burying powdercomprising at least one micrometer sized Li-oxide garnet.
 6. The methodof claim 1, wherein the nitrate dopant source is at least one of a metalnitrate, or a mixture of at least two different metal nitrates.
 7. Themethod of claim 1, further comprising: adding from 1 wt. % to 20 wt. %excess of a nitrate source containing Li to compensate for Li lossesduring the third heating step.
 8. A method of making a Li-ion conductivecubic garnet, comprising: forming a nitrate source aqueous solutioncomprising a first nitrate source and a nitrate dopant source;contacting the nitrate source aqueous solution and a carbohydrate sourceand heating in a first heating step to form a nano-particle ash; asecond heating step of the nano-particle ash to produce a Li-ionconductive pure cubic garnet powder; milling the cubic garnet powder toproduce a sub-micron powder; pelletizing the sub-micron powder to form agarnet pellet; and a third heating step of the garnet pellet to form adense Li-ion conductive cubic garnet membrane.
 9. (canceled)
 10. Themethod of claim 8 wherein: the first nitrate source comprises a mixtureof LiNO₃, La(NO₃)₃, and ZrN₂O₇; the nitrate dopant source is at leastone element from Groups IIA to VIIA, and IIIB to IVB; the carbohydratesource has from 5 to 10 carbon atoms; the cubic garnet powder comprisesirregularly: shaped, porous particles before milling; the sub-micronpowder comprises sub-micron-sized particles having a size in a range of100 nm to 1000 nm after milling.
 11. The method of claim 8, furthercomprising: adding from 1 wt. % to 20 wt. % excess of a nitrate sourcecontaining Li to compensate for Li losses during the third heating step.12. The method of claim 8, wherein the dense Li-ion conductive cubicgarnet membrane has a Li-ion conductivity in a range of 0.2×10⁻³ S/cm to2.0×10⁻³ S/cm. 13-15. (canceled)
 16. The method of claim 1, wherein amole ratio of the nitrate source aqueous solution to the carbohydratesource in the contacting step is in a range of 1:1 to 1:4.
 17. Themethod of claim 16, wherein the first heating step is conducted at atemperature in a range of 200° C. to 550° C.
 18. The method of claim 1,wherein the second heating step is conducted at a temperature in a rangeof 650° C. to 700° C. to form nano-sized garnet nanoprecursor particleshaving a size in a range of 10 nm to 100 nm.
 19. The method of claim 1,wherein the third heating step is conducted at a temperature in a rangeof 950° C. to 1200° C. for a time in a range of 0.5 hr to 30 hrs. 20.The method of claim 19, wherein the third heating step is held at 800°C. for a time in a range of 2 hrs to 6 hrs to form and stabilize a purecubic garnet phase.
 21. The method of claim 8, wherein a mole ratio ofthe nitrate source aqueous solution to the carbohydrate source in thecontacting step is in a range of 1:1 to 1:4.
 22. The method of claim 21,wherein the first heating step is conducted at a temperature in a rangeof 200° C. to 550° C.
 23. The method of claim 8, wherein the secondheating step is conducted at a temperature in a range of 775° C. to1000° C. to form Li-ion conductive pure cubic garnet powder havingirregularly-shaped micron-sized particles.
 24. The method of claim 8,wherein the third heating step is conducted at a temperature in a rangeof 950° C. to 1200° C.